The present invention relates to an alarm system which produces an alarm signal when it detects an intruder in an office, factory, and the like (e.g. the alarm system reports such event to a security company). In particular, the present invention relates to a measure for ensuring transmission of such an alarm signal, even if an intruder is detected during a communication failure or the like.
Regarding an alarm system which produces an alarm signal on detecting an intruder in an office, factory, and the like, a conventional system is equipped with a transmitter which generates a detection signal on detection of an intruder in its monitoring area, and a receiver which reports to a security company (i.e. produces an alarm signal) on receiving the detection signal from the transmitter.
The transmitter is a passive infrared sensor or an active infrared sensor (infrared beam interruption sensor). The passive infrared sensor detects far-infrared rays emitted from a person who enters its monitoring area, and produces a detection signal when an amount of detected rays exceeds a predetermined threshold level. For another, the active infrared sensor is provided with an emitter containing an emitting element and a receiver containing a receiving element. With the emitter and the receiver being disposed face to face across a monitoring area, the emitter projects a near-infrared beam toward the receiver. If the near-infrared beam directed from the emitter to the receiver is interrupted by an intruder, and thus an amount of beam received by the receiving element changes, the sensor produces a detection signal.
There is a different type of alarm system which utilizes a polling system. As illustrated in FIG. 3, a plurality of active infrared sensors 4, 4, 4 is provided such that a detection signal can be transmitted between respective infrared sensors 4, 4. According to the polling system, when any of the infrared sensors 4 detects a person and produces a detection signal, the signal is successively transmitted to the infrared sensor(s) 4 located downstream-along a direction of transmission. Finally, from the infrared sensor 4 which is located at a downstream end along the transmission direction, the detection signal is sent to a receiver 2.
Concerning the above-mentioned alarm systems, however, if a transmission circuit of the transmitter fails or if it is adversely affected by a noise while transmitting a detection signal, the detection signal may not be received by the receiver. Besides, in the case of the polling system, the transmission line of a detection signal is interrupted, if a communication failure occurs temporarily in one of active infrared sensors, or if any of the active infrared sensors is temporarily suspended in an artificial manner. As long as the transmission line remains interrupted, any detection signal produced by a sensor which is located upstream of an interrupted position in the transmission line fails to reach the receiver.
In this situation, by the time a normal communication state is recovered between the transmitter and the receiver, it is often a case that the transmitter has finished transmission of a detection signal. Under such circumstances, the receiver never recognizes that the transmitter detected an intruder. To summarize, if a person enters a monitoring area during communication failure, and the like, the alarm system cannot recognize the intruder and cannot ensure a reliable operation.
The present invention is made in view of such drawbacks. With regard to an alarm system composed of a transmitter and a receiver, and producing an alarm signal when the receiver receives a detection signal from the transmitter, the present invention intends to provide an alarm system in which an alarm signal can be produced with certainty even in an event of a communication failure between the transmitter and the receiver.